There was even iron works and locomotive factory in central Berlin during that time. The area was aptly nicknamed "Feuerland".
A bit further north there was an extremely densely populated residential neighbourhood. Factory workers were living there in terrible conditions. One building (5 floors) had over a thousand tenants. Usually there was only one bathroom for the whole floor. And there was no hot water.
Crime and pollution was a huge problem. The architecture was great, at least compared to post-war times, but life in the city was quite crap.
1.) The period 1871-1873 of rapid economic growth in the German empire following the peace treaty that ended the war of 1870. It ended with a stock market crash (aka Gründerkrach).
2.) More generally the period of broad industrialization from 1870 to 1914.
3.) Somewhat confusingly for architecture it's applied to buildings of the late 1800s through about 1900.
Personally I always understood it as #2 and #3. My in-laws lived for many years in Kassel, which has a lot of houses from that era in the upper part of town around Wilhelmshöhe.